The number of natural numbers between 18 and 78 both excluded which are divisible by 6 is ______.

Correct option is D

Given:

Numbers between 18 and 78 divisible by 6

Formula Used:

The nth term of an AP is given by:

​$$a_n = a+(n-1)d$$​​

Solution:

Here numbers between 18 and 78 divisible by 6 are

24,30,36..72

So the above series is in an AP with first term 24 and common difference 6

Last term is 72

Hence the number of terms:

72 = 24 +(n-1)6

6(n-1) = 48

(n-1) = 8

n = 9